Fix vec_is() by comparing ptypes

[lionel-]

We could make vec_is() generic by requiring the result of vec_ptype() to only contain essential attributes. Methods for vec_ptype() would need to remove any details attributes. Ideally they'd just return a static ptype. vec_is() would then compare the ptypes with identical().

vec_ptype() is already generic, although it is currently documented as a only a performance generic. Here we are considering giving it a specific role in the genericity API of vctrs.

[JosiahParry]

This would be a very helpful change. If the current function can be written as an s3 generic's default method that would be huge from an rextendr perspective. Doing so would make it possible to make externalptr's be compatible with tibbles without doing a compete ALTREP for every type. The below link illustrates an example where we can make an object compatible with a data.frame but not with a tibble due to vec_is.

https://github.com/JosiahParry/vctrsrs#problem-pointers-arent-vectors